Satellite assessment of divergent water vapor transport from NCEP, ERA40, and JRA25 reanalyses over the Asian summer monsoon region

The divergent component of water vapor transports was constructed using evaporation, precipitation, and total precipitable water estimated from the Special Sensor Microwave Imager (SSM/I). The SSM/I moisture budget parameters were then compared with those from the National Centers for Environmental Prediction (NCEP), the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA40), and the Japanese 25-year Reanalysis Project (JRA25) data over the Asian monsoon region for the May to September (MJJAS) period from 1988 to 2000. The climatology of SSM/I water vapor transports clearly indicates three major water vapor sources for the Asian monsoon, i.e., the subtropical Indian Ocean and Pacific Ocean in the Southern Hemisphere, and the North Pacific Ocean. In contrast, sinks are located in the Asian summer monsoon trough, the equatorial convective zones over the Indian and western Pacific Oceans, and over the East Asian monsoon region from the northern tip of Philippine Sea to the Kuroshio extension region. These sources and sinks are linked to well-known large-scale rotational circulation features, i.e., the cross-equatorial flow associated with monsoon circulation over the Indian Ocean, the anticyclonic circulation along the western periphery of the western North Pacific High, and the cross-equatorial flow north of New Guinea. In conjunction with the fluctuation of these sources and sinks, the northward propagating climatological intraseasonal oscillations in water vapor flux convergence are evident in the South and East Asian monsoon regions in the SSM/I data. From the comparison of water budget parameters of NCEP, ERA40, and JRA25 reanalysis with SSM/I-derived features, we found that the general features of all three reanalyses are in good agreement with those from SSM/I; however, the magnitudes of water vapor transports are comparatively weaker in all three reanalyses than what SSM/I measurements suggested. In addition, much weaker water vapor transports in three reanalyses are found in the intraseasonal oscillation signals with less distinct patterns, compared to what are inferred from the SSM/ I measurements.